This invention relates to electron beam cold hearth refining of metals such as titanium alloys which must be completely free of unrefined inclusions and, more particularly, to a new and improved electron beam cold hearth refining furnace which is especially adapted to prevent contamination of refined metal.
In certain applications wherein metals such as titanium alloys which have been refined by electron beam cold hearth refining are used in aircraft engine parts, the presence of even the tiniest amounts of unrefined inclusions in the refined ingot is severly detrimental. Since such inclusions may, for example, result in fracture and disintegration of aircraft engine parts rotating at very high speed they should be completely avoided.
In conventional electron beam cold hearth refining of metals such as titanium alloys, a water cooled hearth is supplied with lumps or pieces of titanium sponge or machine turnings of titanium alloy consisting of scrap from the manufacture of titanium alloy parts. This material is introduced by gravity feed at one end of a cooled elongated hearth in an electron beam furnace in which the material is melted and refined by electron beam impingement. The refined molten material is poured from the opposite end of the hearth into a cylindrical mold where it forms a vertically disposed cylindrical ingot that is withdrawn downwardly within the mold as it solidifies.
In conventional electron beam cold hearth furnaces used for refining of titanium alloy or the like, the raw material often includes vaporizable contaminants such as chlorine in titanium sponge and oil or moisture in machine turnings. As such materials are introduced into the melt area of the hearth and are heated by the molten metal and by an electron beam, the vaporizable contaminants frequently produce relatively violent eruptions in the molten metal being refined. Such eruptions have been found to cause both molten and unmelted material from the melt area to be spattered toward other areas of the electron beam furnace including the casting area where the refined ingot is being molded. As a result, it is possible that unrefined metal containing undesirable inclusions such as titanium nitrides or tungsten carbides, for example, is introduced into the mold and thereby incorporated into the cast ingot and into any final product produced from the ingot, such as a jet engine compressor disc, for example.
Heretofore the provision of a vertical shield over the molten material at the end of the hearth adjacent to the casting area has been proposed in order to block such spattering of material into the mold. With such arrangements, however, unmelted material spattered by eruptions and prevented by the shield from entering the casting area directly can be deflected downwardly from the shield into the molten material at the point where it passes from the hearth into the mold. Furthermore, vaporized material and spattered molten material may accumulate and solidify on the shield and occasionally portions of such solid material containing contaminating inclusions may drop from the shield into the refined molten material as it passes from the hearth into the mold.
Accordingly, it is an object of the present invention to provide a new and improved electron beam cold hearth refining arrangement which overcomes the abovementioned disadvantages of the prior art.
Another object of the invention is to provide a new and improved electron beam cold hearth refining furnace which provides greater assurance that refined metal will be free of undesirable inclusions.